The telecom technology is currently migrating towards an “all IP” infrastructure. There are economical advantages in maintaining a package-based network only. This could result in solutions where today's real time applications are transported over package bearers like i.e. IP, MPLS etc. However, Package based networks were designed for data transmission and not for real time traffic. Important real-time parameters to consider are throughput, jitter (variable delay), losses etc.
The main problem by transporting real time traffic over package networks is to ensure that the quality of the speech data is at least as good as today's solution. The use of e.g. voice over IP involves many protocols (see FIG. 2) each adding a lot of overhead, thus making the transport inefficient. Jitter and packet-loss will degrade the signal quality, as well as overall delay. Jitter can arise when packets take different routes through the network, as may occur in IP networks. This may also cause packets arriving in wrong sequence. Another challenge is to secure sufficient available resources between the two communicating hosts (this is certainly no problem on circuit switched lines, where a constant sufficient capacity is always granted). One method to achieve proper quality is to use MPLS.
Unlike typical routing, MPLS works on the idea of flows, or Forwarding Equivalence Classes (FECs) in MPLS parlance.
Flows consist of packets between common endpoints identified by features such as network addresses, port numbers, or protocol types. Traditional routing reads the destination address and looks at routing tables for the appropriate route for each packet. Each router populates these routing tables by running routing protocols—such as RIP, OSPF, or Border Gateway Protocol (BGP)—to identify the appropriate route through the network.
By contrast, MPLS calculates the route once on each flow (or FEC) through a provider's network. The MPLS-compatible router embeds a label consisting of short, fixed-length values inside each frame or cell. Along the way, routers use these labels to reduce look-up time and improve scalability.
MPLS provides transmission of all packets over the same path (LSP) through the network, thus avoiding packets out of order and jitter. Each LSP can be set up with different QoS parameters.
The MPLS Protocol is shown in FIG. 1. The overhead (10) is 4 bytes and consists of the fields Label (20) (20 bits), Exp (30), (3 bits), S (40) (1 bit) and TTL (50) (8 bits). EXP means EXPerimental bits and may be used when mapping traffic classes from i. e. IP Differentiated Services field (ToS). The S bit (40) indicates Stack depth of MPLS. When S is set, it means that this is the innermost stack. TTL means Time To Live and is adapted from the Ipv4 indicating how many hops the packet is allowed to travel before it is being terminated.
The standard way of using MPLS is to use it in addition to various protocols, just adding more packet overhead for control information. The application type decides what the rest of the protocol stack looks like. Real time traffic has other requirements than non-real-time traffic. Voice traffic may use a protocol stack (100) as shown in FIG. 2.
All protocol layers have their functionality, and they are all adding extra information to the payload, enabling the equipment to route the traffic the correct way and assign priority.
Based on the protocols and header information, the MPLS label will have a bit-field, setting the right priority for the current packet.
One common problem for all the protocols is that the data link is using a lot of capacity just for transferring overhead information. An overhead of 20% or more is possible for real time traffic, when using the protocol stack shown in FIG. 2. This gives a poor utilisation of the network capacity.
Consider for example a packet with user payload of 256 bytes, which could be a typical packet size for real time traffic and requirements regarding delay, as it will be for voice traffic. After the protocol overhead has been added, the packet has increased to 312 bytes, which will be the payload for the SDH/SONET/ATM/Ethernet packet. This means that the packet size has increased by 21.9%.